Occasionally, r5r will generate a travel times matrix with an unroutable destination in the middle of an otherwise routable area. In the case of census tracts, this will appear as a grey (missing value) tract in the middle of an otherwise complete isochrone.
Destinations can be unroutable for a variety of reasons, including:
Issues snapping the destination point to the OSM road network. Rafa's team discovered there is effectively a 1.6 km limit to the search space R5 will use for snapping. They are creating a PR to make this variable user-controlled.
Road types? I'm not positive that this is true, but a cursory look at unroutable destinations made me think some of them landed on highways, which may not be walkable according to R5.
Island road networks. I'm pretty sure R5 does things to clip/remove road network islands, but this was definitely an issue when using OTP.
Bad centroids. Destinations are the centroids of polygons. However, not all census geometries are convex. You can find rare cases where concave geometries put your destination centroid in the middle of a lake. Population weighting your centroid (somewhat) fixes this issue, assuming there is a population to weight by.
We should investigate and try to fix each of these potential issues. However, my experience with OTP tells me that we'll likely always have some gaps in our output matrices. Rafa's team suggested taking an average of touching destination polygons to fill in gaps.
Occasionally, r5r will generate a travel times matrix with an unroutable destination in the middle of an otherwise routable area. In the case of census tracts, this will appear as a grey (missing value) tract in the middle of an otherwise complete isochrone.
Destinations can be unroutable for a variety of reasons, including:
We should investigate and try to fix each of these potential issues. However, my experience with OTP tells me that we'll likely always have some gaps in our output matrices. Rafa's team suggested taking an average of touching destination polygons to fill in gaps.